Polyether polyols are typically synthesized by polymerization of one or more alkylene oxides in the presence of an alkali metal catalyst, such as potassium hydroxide, and a hydroxyl-containing initiator, such as glycerin. Following polymerization, the alkali metal catalyst residue is routinely removed from the polyether polyol by any of a number of methods, including adsorption onto magnesium silicate, or extraction of the alkali metal into water. The water wash generated in the latter process typically contains traces of organic solvent and polyether polyol in addition to the alkali metal catalyst. The pH of this stream is usually greater than 7.
The wastewater stream described above presents a unique treatment problem that is difficult to solve with conventional water treatment solutions. Organic impurities are traditionally removed from wastewater in many different ways, including membrane separation, adsorption, biodegradation, wet chemical oxidation, or a combination of methods. These methods are generally effective for erradicating low molecular weight impurities such as chlorinated hydrocarbons and phenolics. Most of the methods, however, are ineffective for treating wastewater derived from polyol synthesis processes. Polyols containing recurring oxypropylene and/or oxyethylene repeat units, especially those of high molecular weight, are not readily biodegradable. The polyols are often substantially water insoluble, especially those with relatively low oxyethylene content, but they are readily emulsified or suspended in the aqueous stream and are therefore difficult to separate. Wet chemical oxidation of polyether polyol-containing wastewater has not been an economically attractive option for treatment of such wastes, largely due to the fact that the waste streams are very dilute, and concentration is energy intensive.
The primary method of disposal of polyether polyol aqueous waste streams is currently deep-well injection. Since polyols do not biodegrade readily, there is concern that polyols disposed of in deep wells may tend to accumulate. The ecological consequences of such accumulation are unknown. It is reasonable to assume that government regulations concerning the volumes of wastes allowed in deep wells will continue to become more strict. There is a need to find alternative means for treating wastewater.
In Box et al U.S. Pat. Nos. 3,823,088 and 4,072,608, teach wet oxidation processes catalyzed by zinc orthotitanate or zinc aluminate and promoters for treating wastewater contaminated with organic compounds. The organics treated consisted of paraffins, olefins, aromatics, and numerous oxygenated compounds. Oxidation of polyether polyol-containing wastewater is not taught.
Imamura et al. (Ind. Eng. Chem. Res. 27 (4) 718 (1988)) teach a method of oxidizing water-soluble organic compounds, including polyethylene glycol (200 mol. wt.) and polypropylene glycol (1,000 mol. wt.), using a catalyst of ruthenium supported on cerium(IV) oxides. Imamura et al. also teach (Bull. Chem. Soc. Jpn. 54 (5) 1548 (1981)) wet oxidation of water-soluble polymers to greatly improve biodegradability, and wet oxidation of polyethylene glycol using a manganese/cerium composite oxide (Ind. Eng. Chem. Prod. Res. Dev. 25 (1) 34 (1986)). Oxidation of water-insoluble polyether polyols or high molecular weight polyether polyols is not taught.
Hagiwara, et al teach methods of oxidation of wastewater containing water-soluble detergents and surfactants, including polyoxyethylene nonyl phenyl ether and polyethylene glycol (Chem. Abstr. 94 213735w, Chem. Abstr. 96 109583a, Chem. Abstr. 89 79634d). Treatment of wastewater containing water-insoluble polyether polyols is not taught.
Wet oxidation methods for treatment of wastewater containing polyether polyols represent an environmentally acceptable alternative to deep-well injection of these wastes. There is a need for oxidation methods that are effective for degrading substantially water-insoluble and high molecular weight polyether polyol impurities. Methods capable of degrading polyether polyols containing mostly oxypropylene units are needed. Methods capable of degrading polyether polyol impurities in the 2,000 to 20,000 molecular weight range to molecular weights substantially below 1,000 are needed. Since a major proportion of commercially important polyether polyols have molecular weights from about 2,000 to 6,000, methods for degrading these are especially needed.